Automotive seat tracks which are adjustable four ways or six ways are very well-known and constitute a well-developed art. Adjustments may be manual or automatic (motor-driven) as shown by many patents such as U.S. Pat. No. 4,015,812 issued Apr. 5, 1977 to M. Heesch.
In the cited patent, a multiple shaft motor is used to generate the three adjustments, fore and aft, front end vertical and rear end vertical. Each motor shaft drives a gear train within a respective gear box to rotate a lead screw. Each lead screw mates with its own drive block. For the fore and aft drive, rotation of the fore and aft lead screw causes relative translatory movement between the drive block and the gear box. Since one (block or gear box) is affixed to the stationary seat track and the other to the movable track, translation of the movable seat track in the fore and aft direction results from rotation of the fore and aft lead screw. For vertical adjustment, the respective drive blocks are each secured to one bracket arm of a bell crank, the other end of each bell crank being secured to the respective end of the seat to control the elevation thereof.
In this structure, each drive block is pinned on both sides to the bracket members, the pins entering the drive block for a short distance to prevent interference with the mating lead screw. Since all the driving forces must be transferred from lead screw to drive block, and from the drive block mounting to the adjacent bracket, the pin connection members must withstand the mechanism driving forces.
For this reason, the rivets or pins are serrated for press fit relationship with the adjacent bracket member. The pins must act to hold the bracket member from spreading outwardly since the brackets are essentially U-shaped with the drive block supporting members being the legs or sides of the U. Such brackets have a tendency to spread when shock loading is applied and the pins must act to lessen this tendency of the bracket legs to spread.
For the same reason, the pins must be secured within their block and must be secured to the adjacent bracket. The assembly of the pins to the drive blocks and the drive blocks to the seat track therefore becomes an expensive and complex operation.
Generally stated, the drive block or nut is used to position the upper track of an automotive power seat track assembly relative to the lower track via the drive screw's rotary motion in the nut. Space requirements dictate a "U" shaped bracket to keep the nut from rotating which limits the strength of the assembly. Pins have been inserted in the bracket and nut as one means of holding the nut in position. Screws have also been used for this purpose.
Actual strength of the pinned nut assembly is widely variable since if the pin is inserted in the nut too far, destructive forces are induced in the nut which contribute to its failure. If the pin is not in far enough, the holding power of the pin is reduced. The screw-nut assembly has a similar problem in that the assembly is weakened if the screw threads are partially stripped due to overtightening. Undertightening results in a weaker assembly due to decreased thread engagement. All of these assembly conditions are subject to day to day assembly variations.
When the nut-bracket assembly is subjected to the large forces that are required of seat tracks for safety reasons, the "U" shaped bracket opens; and in the case of the pins, disengages the pins from either the nut or the bracket and frees the seat. When screws are threaded into the nut to hold the "U" shaped section together, the forces are of such a magnitude as to shear the threads in the nut material. Increasing the screw thread engagement is prohibited because the space allowed for the nut-bracket assembly is minimal. Economic and weight considerations do not allow additional nut or bracket material to be added.